This application relates to bonding of metal films to semiconductor and other substrates, particularly to bonding of metal films to substrates using metallic adhesives, and more particularly to bonding while controlling the surface passivation caused by such metallic adhesives.
Conductive metals are commonly used in integrated circuits and other semiconductor devices to form a variety of structures including the conductive interconnects between semiconductor elements, and the wire bond pads used to connect the device to the chip carrier. Many of these conductive metals, however, have poor adhesion to the substrates commonly used in the industry. Accordingly, bonding of metal films to semiconductor substrates is a widely popular technique in the manufacture of integrated circuits. More recently, bonding of metal films to polyimide and other polymer substrates has become a popular technique used in the manufacture of flexible printed circuit boards and other devices.
A variety of techniques are commonly employed to bond the films to the substrates. Common techniques include surface roughening, irradiation, sputtering, use of chemical modifiers, and use of interfacial metallic adhesives. Of these, use of interfacial metallic adhesives is probably the most popular and the most practical method. Metals employed as metallic adhesives are oxygen-active metals that also have a solid solubility in the conductive metal film to be adhered. Such metallic adhesive materials include chromium, titanium, and aluminum. Use of these oxygen-active metallic adhesives, however, has led to the problem of degradation of the conductive film surface due to grain-boundary diffusion of the metallic adhesive to the surface of the conductive film.
Ordinarily, to maximize the area for bonding, the metallic adhesive is applied to the substrate in a pattern that substantially matches the pattern of the metal film to be attached and the conductive metal film is applied over the metallic adhesive. When the device is exposed to oxygen, the metallic adhesive migrates through the metal film and forms oxides on the surface, which passivates substantially all of the surface of the metal film. The passivation process is accelerated by temperature. Usually, electronic devices rely on the bulk conductive properties of the metal films applied to the substrate, so the surface condition of the metal film after the device is fabricated is of little consequence and a passivated surface may even be desirable. For many applications, however, it is important to avoid passivation of the surface of the metal film. Control of surface passivation is particularly important for metal-film chemical sensors, which rely on a surface reaction of the metal film to produce a response.
Examples of such chemical sensors include the apparatus disclosed in U.S. Pat. No. 3,714,562 to McNerney (the '562 patent) in which a chemical sensor composed of a gold film on an insulating substrate selectively adsorbs mercury to cause a gradual change in the resistance of the gold film. The chemical switch disclosed in the aforementioned co-pending application Ser. No. 08/031,610, now U.S. Pat. No. 5,466,605, incorporated herein by reference, comprises a noble metal film on an insulating substrate that selectively adsorbs chlorine or other reactive components to cause an abrupt change in the resistance of the film. In these and similar devices, it is desirable to achieve satisfactory adhesion of the device to the substrate with minimal passivation of the active surface of the metal film. Otherwise, the surface passivation will eventually render the device incapable of adsorbing the target chemical.
Heretofore, metal-film chemical sensors have used ordinary metal film adhesive techniques to attach the sensor elements to their substrates and, as a consequence, have had a limited service life. A gold film sensor of the type disclosed in the '562 patent used to detect mercury must be heated after a sample is taken to desorb the adsorbed mercury species and thereby regenerate the sensor for another cycle of operation. Each time the sensor is heated, however, the passivation of the sensor is exacerbated. Frequently, such sensors are rendered unusable after fewer than a few hundred cycles. What is needed then, is a method of attaching metal films to substrates that provides adequate adhesion for structural integrity, yet eliminates the adverse surface passivation effects of the metallic adhesives.
Accordingly, it is a principal object of this invention to provide a method for attaching metal films to substrates with metallic adhesives that provides adequate adhesion while eliminating the adverse surface passivation effects of the metallic adhesive. A further object of this invention is to provide a chemical sensor having an active element and a reference element fabricated by the same process steps as the active element, but having a purposefully passivated surface.